1. Field of the Invention
The present invention relates to a display system having a keystone distortion, and more particularly, to an apparatus and a method for correcting keystone distortions of a display system using format conversion.
2. Background of the Related Art
In general, big screen projection televisions, plasma display panel (PDP) televisions, and projectors are in great demand as display devices for digital televisions of these days due to the fact that high quality displays are now available. On the other hand, the demand for formerly used color picture tubes (CPT) having a direct view structure is currently being reduced. Particularly, the projectors have an advantage that they can provide amazingly large display screens compared to other display devices. For that reason, the extent of their uses is sharply increasing: the projectors are frequently being used even in personal digital televisions in addition to their former uses as display devices for business or seminar presentations.
The projectors require keystone correction that may not be required for other display devices. In a typical display device other than the projectors, a scanning unit and a screen unit are located inside of the display device, so frequent keystone corrections are not necessary. However, the keystone correction functions of the projectors are critically essential since the screen is separately located far away from the scanning unit.
When a projector is located above the centerline of the screen, it requires that the projector be aimed down at the screen and produces a keystone distortion causing the image at the bottom of the screen to be wider than at the top. Similarly, the image at the top of the screen will be wider than at the bottom if the projector is located under the centerline of the screen. Accordingly, the projector must initially generate the image having a keystone figure before projecting to the screen in order to have the display on the screen without any keystone distortion. Additionally, it is desired to be able to adaptively correct the distortion when the amount of the distortion varies. Projection televisions do not require various keystone corrections as much as the projectors do, but some correction is necessary as the thickness of a projection television is reduced.
In accordance with the prior art, the keystone correction usually involves in various controls of an optical device such as a lens system. Since it mechanically controls movements of the lens system, it results in increasing the cost of the projectors. It also requires frequent calibrations for an accuracy due to optical errors resulted from frequent keystone corrections. Change of the amount of keystone distortion due to mechanical vibrations or dilapidated parts of the projectors causes another problem to be generated. For offsetting such changes, the amount of the correction also must be controlled in order to have the optimal display quality.